1. Field of the Invention
The present invention relates to an improved vapor phase process for the oxidation of acrolein to acrylic acid using as oxidant particulate molybdenum vanadate multimetal oxide in an oxidized state, and wherein the resulting reduced solids are separately regenerated using molecular oxygen. More specifically but not by way of limitation, the invention relates to a process for performing this reaction in a recirculating solids reactor system.
2. Description of the Related Art
An important route to acrylic acid is the vapor phase oxidation of acrolein over a multicomponent catalyst containing molybdenum, vanadium and/or other metals, usually as their oxides. The reaction step involves oxidation of acrolein with air (oxygen) to form acrylic acid, along with carbon oxides, water and smaller amounts of other oxidized byproducts. Preferably, the feed gas also contains steam. Typically the reaction is carried out in multitubular fixed-bed reactors. The large exothermic heat of reaction and the thermal sensitivity of the acrolein oxidation requires low feed concentrations, expensive heat transfer equipment, handling of a large volume of gas, and good reactor temperature control. Low acrolein concentration is also required to avoid flammability conditions.
The magnitude of some of these problems is reduced when a fluidized-bed reactor is used. The temperature can be readily controlled within a few degrees because of the intensive solids mixing and the good heat transfer characteristics. Higher acrolein concentrations can be used because the danger of flammability is reduced by introducing the acrolein directly into the reactor rather than premixing it with air (oxygen). However, very high acrolein concentrations and low oxygen-to-acrolein ratios in the reactor may result in the over reduction of the solids and reduced selectivity to acrylic acid. Also, significant back-mixing of gases in the fluidized-bed reactor result in selectivity losses.
Modified forms of fluidized-bed reactor which minimize back-mixing are known as recirculating solids reactor, transport bed reactor, transport line reactor, riser reactor, fast fluidization reactor, multi-chamber fluidized bed reactor, and by other names, depending on design and/or personal preference. In this application we will use the term "transport bed reactor" to mean any reactor in which solid particles are injected at one end of the reactor and carried along with gas reactants at high velocities and discharged at the other end of the reactor to a gas-solids separation vessel. A riser reactor, in which the reactor is a vertical pipe wherein the active solids and gases are fed in at the bottom, transported in essentially plug flow and removed at the top, is one example of a transport bed reactor. Another example is a pipeline reactor, in which the flow of active solids and gases is other than vertically upwards. A transport bed reactor, as defined herein, includes a riser reactor or pipeline reactor which also incorporates a zone for fluidization, i.e., a zone where the gas velocities are sufficiently high to carry out a substantial portion of the active solids fed, but with more back-mixing of active solids than would occur in plug flow. We will use the term "recirculating solids reactor system" to mean a general reaction system with two reaction zones, in which two separate reactions take place, and which uses a particulate solid which circulates between the two reaction zones and takes part in both reactions. Optionally, either or both reaction zones may involve either a transport bed reactor or a fluidized bed. Such reaction systems have found use in catalytic cracking in petroleum refining and in other reactions.
U.S. Pat. No. 4,668,802 discloses a process for preparing maleic anhydride by oxidizing butane using an oxidized vanadium-phosphorous oxide catalyst as oxidant rather than oxygen wherein the resulting reduced catalyst is separately regenerated, and the use of a recirculating solids reactor system for this reaction. Certain of the examples use a transport bed or riser reactor for the butane oxidation reaction.
Japanese Kokai 3-170,445 discloses a similar process for preparing a mixture of acrolein and acrylic acid by oxidizing propane using an oxidized bismuth-molybdenum catalyst or vanadium pyrophosphate catalyst as oxidant.
An advertising folder prepared by E. I. DuPont in 1973 titled "Chemical Technologies Worldwide" included a single sheet titled "Transport Bed Reactor Technology for Selective Processes", which described the general advantages of a transport bed or riser reactor, listing among typical applications the reaction of propylene to make acrylic acid.
The preparation of multicomponent compositions containing molybdenum and/or other metals and their use as catalysts in the oxidation of acrolein to make acrylic acid is well known in the art. For example, numerous patents such as U.S. Pat. No. 4,092,354 disclose specific compositions containing molybdenum and vanadium for use in the oxidation of acrolein to acrylic acid in a vapor phase oxidation using molecular oxygen. U.S. Pat. No. 4,677,084 discloses a process for making highly attrition resistant silica-based catalysts containing molybdenum, vanadium or other metals.
None of the above references disclose the necessary information to enable the economical use of a vapor phase process for the oxidation of acrolein to acrylic acid using as oxidant a solid in an oxidized state, and where the resulting reduced solid is separately regenerated using molecular oxygen.